Alkali metals are made from molten salts of the metal in electrolytic cells. This method of making alkali metals has remained largely unchanged for a century. Although some carbothermic-based processes have been proposed, ultimately such processes have been proven to be less economical than the molten salt electrolysis method. The very high activity of these metals usually requires an electrolytic method. Alkali metals are known to be able to be made as amalgams from aqueous systems using mercury as a cathode. However, mercury has the potential to cause severe environmental harm, thus its use has been banned or strictly limited in most developed countries.
Even though the production of alkali amalgams for recovering alkali metals is mentioned in U.S. Pat. No. 2,990,276, U.S. Pat. No. 4,156,635, U.S. Pat. No. 4,455,202, U.S. Pat. No. 4,988,417, U.S. Pat. No. 2,145,404, U.S. Pat. No. 2,234,967, and U.S. Pat. No. 4,156,635, none of these patents teaches the proper combination of chemistry [and alloys so as to create a process that is more economical than the molten salts systems that need to be replaced.
The molten salts systems themselves require difficult conditions, such as the heating of electrically-conductive crucibles (usually made of graphite) to temperatures above that of the molten salt being used, electrolyzing the salt, and collecting the molten alkali metal.
Lithium was first discovered in the early 1800s, via electrolysis of a high-temperature molten salt. Today, lithium is industrially produced in essentially the same way. Major improvements that have been made over the past two hundred years mostly relate to the selection of different types of molten salts that are used as the electrolyte. Careful combinations of the salts have allowed for a decrease in operating temperatures (still several hundred degrees Celsius), and thereby enhanced system stability and lowered operating costs. In the intervening historical period, a low temperature, water based technology was also developed. This process derived from the electrolysis of brine to form chlorine at an anode and sodium hydroxide or potassium hydroxide via a series of cathode related reactions. The formation of either of these hydroxides involves the reduction of the alkali cation to metal at a liquid mercury cathode, followed by reaction of the mercury amalgam so formed with water. This process operates near room temperature and at a lower voltage than is required for molten salt systems. It has long been known that lithium chloride in water will undergo the same chemistry, producing a Hg(Li) amalgam and chlorine gas. It is also well known that other water soluble salts of lithium will also generate this amalgam, but that the anode product will change depending on the anion present in the original lithium compound. Thus, for example, if lithium hydroxide (LiOH) is used as the starting material, oxygen will be formed at the anode.
The Hg(Li) amalgam, once electrochemically formed, will yield pure lithium metal if it is removed from the water electrolyte while still under potential control and then either extracted with an agent, such as an amine, and distilled. While such an approach might produce electrolytic lithium more cost-effectively than the molten electrolyte methods, it would generate unacceptable environmental problems. For example, day-to-day operations would require large amounts of mercury, which, in the event of a mechanical failure of the containment vessel, could leak from a cell and contaminate the environment.”
In order to circumvent these problematic issues, the present invention removes the mercury electrode in the above process and replaces it with a liquid metal alloy electrode. Like mercury, the alloy would be selected so that it had both a high hydrogen over-potential and good chemical kinetics for amalgamation with lithium. However, in contrast to mercury, the proposed metal systems would be solid at room temperature, melting at relatively low temperatures (ideally, at no more than slightly above 100° C., where the water based electrolyte would boil), and would not be highly toxic. Several alloys of bismuth, lead, tin, and indium meet these requirements.
U.S. Pat. No. 4,455,202 (Electrolytic Production of Lithium Metal) Jun. 19, 1984 uses a similar liquid metal cathode in relation to a fused salt electrolyte, which still requires high temperatures, i.e., several hundred degrees Celsius. U.S. Pat. No. 6,730,210 discloses a low temperature alkali metal electrolysis process in the presence of a co-electrolyte and an alkali metal halide. However, in the latter case, the electrolysis process has several problems. First of all, the process requires highly toxic acid and produces a highly toxic halogen gas as a side product both of which are environmentally unfriendly. Second, the solubility's of lithium halides in water are somewhat limited, thus the electrolysis efficiency is not high.
Accordingly, the present invention provides a process and system for the extraction of lithium from lithium carbonate or its equivalent lithium ion source at much lower temperatures using a far more environmentally friendly process.